Telescopic handling arms of known type generally include a plurality of elements of section that is substantially rectangular with rounded corners, which elements are mounted to slide relative to one another, with skids being interposed therebetween.
Generally, slidably-mounted telescopic elements are elements of smaller section that are retractable into stationary elements of greater section.
For aerial lifts for lifting personnel, measuring means are known for measuring their telescoping length, which measuring means measure the extension of the internal or telescopic element.
The measuring means are generally constituted by a system comprising a cable winder, and a rotary encoder that measures the total turning angle of the cable winder. The winder and the rotary encoder are generally secured to the stationary beam of largest size, while the distal end of the winder cable is secured to the movable portion, the movement of which is to be measured.
When the telescopic arm is extended, the extension thus causes the cable to wind out, which in turn causes the winder to turn. The turning of the winder is measured by the rotary encoder that emits a signal to a processing system so as to determine the looked-for length.
Those means for measuring by unwinding a length of cable present several drawbacks. Winding the cable onto the winder generally requires a winder of large diameter, and this constitutes a bulky part to be positioned on the side of the main beam. The cable outside the main beam is also in danger of being damaged or ruptured in operation, or as a result of an undesirable accident or incident.
Other known means consist in placing measuring devices inside the hydraulic telescoping actuator in order to measure the stroke of the actuator. Such devices are usually very costly and limited to actuator lengths that are relatively short.